This invention relates to the field of cathode ray tube degaussing and, more particularly, logic control of degaussing under predetermined conditions.
It is well known in the art of CRT's having three electron beams for color reproduction that small amounts of magnetization in the metal parts of the tube can affect the purity of the colors. A very weak magnetic field near an electron beam can produce a deflection error sufficient to cause the beam to strike phosphor dots for which it was not intended. Many types of arrangements have been used in the manufacture of CRT devices for optimizing color purity including locating permanent magnets in or near the convergence yoke assembly, varying the direction of the magnets in a magnetic field equalizer assembly, and adjusting the deflecting yoke axially. Typically, when a CRT device is moved with respect to the direction of the earth's magnetic field, color purity must be readjusted. Various types of degaussing coils have been placed around the periphery of the face plate or around the conical portion of the CRT for demagnetizing portions of the CRT structure which become magnetized during use. In some devices, manual degaussing of a CRT device is a process to be performed by a service man. Many present day color CRT devices are designed to be degaussed automatically. Since the energizing of the degaussing coil would cause a noticeable flash on the phosphor screen, degaussing is usually done as the CRT device is turned on and before the electron beams are on. This mode of operation is usually satisfactory for apparatus in the home, such as television sets, but in industrial or business environments there may be relatively strong magnetic fields at times due to starting electric motors and the like which severely effect color purity. In some devices, the whole device is shut down manually when degaussing becomes necessary, but an unwanted shutdown may be less desirable than having some color impurity and generally causes loss of data in a data terminal.